1. Preheat grill to 350° to 400° (medium-high). Cut bread loaf into 8 (1-inch-thick) slices; cut slices in half, if desired.
2. Drizzle tomatoes and onion with 2 Tbsp. oil, tossing to coat.
3. Grill tomatoes and onion, covered with grill lid, over 350° to 400° (medium-high) heat 3 to 4 minutes on each side or until lightly charred.
4. Coarsely chop grilled tomatoes and onion, and place in a medium bowl. Add 2 Tbsp. oil, cucumber, and next 3 ingredients. Season with salt and pepper to taste.
5. Brush bread slices with remaining 1 Tbsp. olive oil. Grill, without grill lid, over 350° to 400° (medium-high) heat 1 to 2 minutes on each side or until lightly browned and toasted.
6. Place grilled bread slices on individual serving plates; top with tomato mixture.